The family of fibroblast growth factors (FGF) includes the FGF19 subfamily, which consists of human FGF21, FGF23 and FGF19 and mouse FGF15. Unlike other members of the FGF family, which typically act on their tissue of origin in a paracrine manner, members of the FGF19 subfamily act on specific distal tissues in an endocrine manner. The effects of FGF family members are the result of their heparin-dependent binding to one or more members of the FGF receptor tyrosine kinase (FGFR) family. This family of receptors includes four members each having a tyrosine kinase domain, FGFR1, FGFR2, FGFR3 and FGFR4, as well as two splice variants each of FGFR1, FGFR2 and FGFR3. These splice variants, which occur in exon 3 of FGFR1, FGFR2 and FGFR3, are designated as “b” and “c” variants (i.e., FGFR1b, FGFR2b, FGFR3c, FGFR1c, FGFR2c and FGFR3c, which are also known as FGFR1(III)b, FGFR2(III)b, FGFR3(III)c, FGFR1(III)c, FGFR2(III)c and FGFR3(III)c, respectively).
Members of the FGF19 subfamily have been implicated in regulating a variety of tissue-specific metabolic processes in mammals. Of particular interest is FGF19, which has been shown to target and have effects on both adipocytes and hepatocytes. For example, mice treated with recombinant human FGF19 (rhFGF19), despite being on a high-fat diet, show increased metabolic rates, increased lipid oxidation, a lower respiratory quotient and weight loss. Moreover, such mice showed lower serum levels of leptin, insulin, cholesterol and triglycerides, and normal levels of blood glucose despite the high-fat diet and without appetite diminishment. Obese mice that lacked leptin but included a FGF19 transgene showed weight loss, lowered cholesterol and triglycerides, and did not develop diabetes. Obese, diabetic mice that lack leptin, when injected with rhFGF19, showed reversal of their metabolic characteristics in the form of weight loss and lowered blood glucose. (Fu, L. et al., Endocrinology 145(6), 2594-2603 (2004); Tomlinson, E. et al., Endocrinology 143(5), 1741-1747 (2002)).
Another member of the FGF19 subfamily, FGF21, is expressed primarily by the liver and has metabolic effects similar to that of FGF19, such as increased metabolism via its effects on adipose tissue, weight loss, lowered blood glucose levels, and resistance to obesity and diabetes. (Kharitonenkov, A. et al., J Clin Invest 115(6), 1627-1635 (2005)). FGF21-transgenic mice were also resistant to diet-induced obesity. Moreover, in diabetic rodent models, FGF21 administration lowered blood glucose and triglyceride levels.
FGF21 has been also shown to have a role in regulating the growth hormone (GH) pathway. The anabolic effects of GH are mediated by insulin-like growth factor 1 (IGF-1), which is primarily produced by the liver. GH induces IGF-1 transcription, thereby increasing its circulating levels, via activation of the Janus kinase 2 (JAK2) by the GH receptor. Activated JAK2 phosphorylates members of the signal transducers and activators of transcription (STAT) family which, when phosphorylated, undergo nuclear translocation and bind to regulatory elements of target genes, including those of IGF-1. In particular, STAT5, in its phosphorylated form, has been shown to have a prominent role in this response.
The effects of GH on IGF-1 levels appear to be countered by starvation or fasting—conditions that result in lower levels of IGF-1 transcription and circulating IGF-1. (Thissen, J. P. et al., Endocr. Rev. 15, 80-101 (1994)). These effects on IGF-1 may be due to reduced levels of phosphorylated STAT5. In particular, fasted rats injected with GH have lower levels of hepatic phosphorylated STAT5 than non-fasted rats. (Beauloye, V. et al., Endocrinology 143, 792-800 (2002)). FGF21, which is induced in the liver under starvation or fasting conditions, may mediate this effect. FGF21-transgenic mice have been shown to have lowered levels of IGF-1 and phosphorylated STAT5. (Inagaki, T. et al., Cell Metabolism 8, 77-83 (2008)).
The metabolic effects of FGF19 and FGF21 are effected via their binding to the FGFR1c, FGFR2c and FGFR3c receptors, of which the binding to FGFR1c and FGFR2c are the most significant. Furthermore, binding of FGF19 and FGF21 to these receptors require the co-receptor Klotho-beta. Despite the prevalence of these FGFR receptors, the metabolic effects of FGF19 and FGF21 are made adipocyte-specific due to this requirement for the Klotho-beta co-receptor, which has tissue-specific localization.
FGF19 has also been shown to have effects that are distinct from FGF21. For example, FGF19 has been shown to regulate bile production by the liver via its liver-specific effects. In response to postprandial bile-production, FGF19 negatively regulates bile production by repressing transcription of the cholesterol 7-alpha-hydroxylase gene (CYP7A1), a rate limiting enzyme in the synthesis of bile acids, and by stimulating the filling of the gall bladder. In addition, FGF19 appears to have liver mitogenic effects that are not observed with respect to FGF21. For example, FGF19 transgenic mice develop hepatic adenocarcinoma due to increased proliferation and dysplasia of hepatocytes, and rhFGF19-treated mice exhibit hepatocyte proliferation of hepatocytes. (Nicholes, K. et al., Am J Pathol 160, 2295-2307 (2000).)
These additional activities of FGF19 appear to be mediated via its binding to FGFR4. FGF19 can bind FGFR4 in both a Klotho-beta-dependent and -independent manner. Although FGF21 has also been shown to bind FGFR4 in a Klotho-beta-dependent manner, no efficient signaling results from the binding of FGF21 to FGFR4.
There is a need to develop new therapies for the treatment of metabolic-related disorders such as diabetes, obesity, high blood sugar, and other related disorders. There is also a need to develop new therapies for such metabolic-related disorders in which the undesired growth or proliferation potential (e.g., tumorigenic potential) of such a therapy is eliminated or reduced. There is also a need to develop new therapies for such metabolic-related disorders in which the potential for growth hormone resistance of such a therapy is eliminated or reduced.